True food allergy is less common than food intolerance, but has increased dramatically in frequency during the last 10 years. It is estimated that around 1.5% of adults and 7% of children suffer from food allergy. It is the commonest cause of anaphylaxis (a life-threatening allergic reaction) in children. Peanut allergy is the most common cause of fatal or near-fatal reactions, and has trebled in incidence over the last four years. It is estimated that one in 70 children in the UK suffers from peanut allergy. Other common food allergies include milk, eggs, prawns and wheat. What causes food allergy?

The word allergy is often used loosely to describe any unpleasant reaction to a substance or a food. However, it should only be used to describe a reaction produced when the body is exposed to a normally harmless substance, against which the body has previously manufactured an antibody called IgE. True food allergy can therefore not occur on first exposure to a food. The antibody production is triggered by one exposure, and then the allergic reaction happens on subsequent exposure. The reaction between the food and the specific IgE against that food leads to the release of a number of chemicals, including histamine. The allergic response is very rapid, usually within a few minutes of eating the offending food. Common symptoms include itching, swelling of the tongue, lips and throat, skin reactions and breathing difficulties. Abdominal pain and vomiting may also occur. The severity of the symptoms usually increases with repeated exposure. Food intolerance

Intolerance can produce a variety of unpleasant symptoms after eating a substance that your body can't handle, often because the digestive system isn't producing enough of a particular enzyme or chemical needed to break down and digest the food. A good example of this is lactose intolerance (lactose is a sugar found in milk) due to deficiency of the enzyme lactase. This leads to milk intolerance, which is quite separate from milk allergy (which occurs as an allergic reaction to cow's milk protein). Food intolerance most often produces digestive symptoms such as bloating, wind, abdominal pain and diarrhoea.

What about allergy testing?

Some allergy tests can be arranged via GPs. A blood sample can be tested to measure specific IgE levels, if the type of food causing the allergic reaction is known or suspected from the history. Skin-prick testing allows up to 25 allergens to be tested for at one time, so is more useful if there is more doubt about what is causing the allergy. This is most commonly undertaken in a hospital clinic. A small drop of a solution containing a single allergen is placed on the skin, and the skin is pierced with a needle. This is repeated on different areas of skin, using different solutions. Any allergic reaction that develops in the skin is noted 15 minutes later. What about other tests?

Applied kinesiology, hair analysis and vega testing are test methods that are widely offered by non-medical practitioners. There is no scientific basis for these tests, and they are considered by conventional medical practitioners to have no place in the diagnosis of true allergy. There is controversy surrounding IgG testing. There is a theory that some cases of food intolerance are due to a 'delayed food allergy', and that this is associated with raised levels of IgG (as opposed to the previously mentioned IgE) in the blood. It is possible to send a blood sample to a private laboratory for IgG measurement, but the validity of this test is the subject of much debate amongst conventional medical practitioners. The role of IgG antibodies in food intolerance, and the value of IgG testing is still unclear.